powerpc/chrp/pci: Make some functions static

These functions can all be static, make it so. Fix warnings treated as
errors with W=1:

  arch/powerpc/platforms/chrp/pci.c:34:5: error: no previous prototype for ‘gg2_read_config’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/chrp/pci.c:61:5: error: no previous prototype for ‘gg2_write_config’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/chrp/pci.c:97:5: error: no previous prototype for ‘rtas_read_config’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/chrp/pci.c:112:5: error: no previous prototype for ‘rtas_write_config’ [-Werror=missing-prototypes]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>